The present invention relates generally to devices and methods for use in assessing various mechanical properties of tissue and, more particularly, to devices and methods for assessing autologous pericardium for potential use in repairing or reconstructing heart valves.
The use of autologous tissue and particularly autologous pericardium for repair and reconstruction of heart valves is increasing. Surgeons involved in the use of autologous pericardium for valvular surgery, however, have reported a significant incidence of the need for re-operation. Although the need for re-operation has not been directly correlated to the use of defective pericardium, without sufficient pre-use testing of the tissue it is not possible to be certain that the quality of the pericardium has not been a factor in those patients who require re-operation for failed valves.
The use of pericardium in heart valve reconstruction requires that a two-dimensional tissue pattern be transformed into a three-dimensional semilunar valve, having a preferred trefoil pattern as disclosed in U.S. Pat. No. 5,719,399, the contents of which are hereby incorporated herein for all purposes. Additionally, pericardial reconstruction of a heart valve depends upon the biomechanical and physical properties of the tissue to ensure an appropriate strength and deformation of the leaflets during the normal cardiac cycle. Properties of crucial importance for ensuring normal valve action are thickness, tensile stiffness, and anisotropy. Anistropy is the measure of stiffness of material as the material is pulled in two orthogonal axes. Materials that have equal stiffness in both axes are referred to as isotropic, while materials with statistically different values are referred to as anisotropic. The importance of tensile strength to valve action is obvious: the leaflets need to support peak pressure loading. Additionally, since each leaflet of the trefoil will be oriented 120 degrees from one another, the radial and circumferential strength of the tissue needs to be as uniform (i.e., isotropic) as possible to ensure equivalent action of each leaflet of the valve.
The gross appearance of the pericardium, and any past medical history of pericarditis or collagen disease, will form the basis of the surgical decision to use or not to use the patient""s own tissue. Objective criteria, however, would greatly assist the decision making process. A knowledge of the basic mechanical properties of the pericardium may be helpful in identifying tissue that is not suited for valvular surgery. For example, if the tissue used in the reconstructed heart valve is too anisotropic, the leaflets may close asynchronously, affecting the functionality of the valve. Eliminating unsatisfactory pericardium before it is used might improve the success of valvular surgery with autologous tissue.
Conventionally, certain mechanical properties of tissue and similar structures were analyzed using uniaxial and/or biaxial test methods. Current laboratory methods and equipment, however, are not practicable for intraoperative use because the test equipment is overly large, requires regular maintenance and calibration, and because of the time required for conducting the tests. Additionally, it is generally impractical to run laboratory-type uniaxial tests in the operating room during a surgical procedure. Furthermore, uniaxial tensile tests, which were previously employed in many instances, are generally destructive to the tissue. Presently, surgeons desirous of using autologous pericardium face a significant challenge in incorporating quality control in the operating room where there are constraints imposed by time and the need for sterility.
The technique of inflating circular disks of tissue to investigate the anisotropy of the tissue has been demonstrated in the past. See, e.g., Zioupos, P., Barbenel, J. C., and Fisher, J., Mechanical and optical anisotropy of bovine pericardium, Medical and Biological Engineering and Computing, January 1992, 76-82; and Zioupos, P., Barbenel, J. C., and Fisher, J., Anisotropic elasticity and strength of glutaraldehyde fixed bovine pericardium for use in pericardial bioprosthetic valves, Journal of Biomedical Materials Research, Vol. 28, 49-57 (1994). The contents of both of these articles are incorporated herein for all purposes. However, the apparatus described therein to inflate the tissue and to determine the strength of the tissue were developed for use in the laboratory and are generally unsuitable for use in the operating room.
Accordingly, there is a continuing need for improved methods and devices for assessing various mechanical properties of tissue, such as pericardium that may be used for heart valve repair, reconstruction, or bioprostheses construction. A single preferred device would assess the thickness and strength of the tissue and would determine if it was isotropic or anistropic. Desirably, the novel device would be small, handheld, sterile, and disposable to allow for rapid measurement of these properties.
The present invention provides a self-contained device for evaluating mechanical properties of tissue. The device and methodology are based upon both optical and mechanical principles and are designed to measure, evaluate and assess critical mechanical properties of tissue, such as autologous tissue to be used for valvular repair or reconstruction. The compact, sterile, disposable handheld device is preferably designed to measure the thickness, strength and any inherent anisotropy of the tissue. The preferred method is non-destructive to the tissue.
The device measures the thickness of the tissue using a mechanical thickness float resting on a portion of the tissue. The float include reference marks that may be aligned with another set of reference marks on the device, forming a vernier scale. In order to assess tissue strength and isotropy, a portion of the tissue is secured within a small circular aperture and inflated. By noting the height of the inflated dome using a mechanical float and vernier scale, tissue strength can be determined. For evaluation of anisotropy, utilization of moirxc3xa9 fringe analysis, a conventional method of interferometry, is employed. Thus, while the tissue is inflated to form a dome, a collimated beam of light, offset from the tissue at approximately 45 degrees, illuminates the tissue. The illumination passes through a pattern of fine parallel lines, or Ronchi ruling. Illuminating and viewing the tissue through the ruling produces an interference pattern on the tissue that can be readily viewed. For a convex dome of a material with perfect isotropy, a pattern of concentric rings is observed. An anisotropic tissue will exhibit a pattern of oval or elliptical rings.